Online games have become popular, wherein a plurality of terminal devices connected to a server device via a communication network progress the video games using game data provided from the server device. Game contents used in online games, such as cards and items, may be traded in video games with virtual currency. For example, Japanese Patent Application Publication No. 2009-187143 (the “'143 Publication”) discloses a trading system for performing, in virtual space, trades of items used in online games. In the trading system of the '143 Publication, items can be purchased with virtual currency, and the conditions of purchase are determined through an auction.
In some online games, game contents can be not only traded but exchanged between players. An exchange of game contents can be concluded when one player offers to another player the exchange of game contents with a certain condition and the other player accepts the offer.